The early developmental expression of cytokeratins will be examined in human embryonal carcinoma cells and cell lines representative of early and gestational trophoblast by immunoprecipitation, immunoblotting, and gel electrophoresis. A cDNA clone for cytokeratin 18 will be characterized by DNA sequencing and RNA analysis. Antibodies specific for different domains of the molecule will be prepared by using synthetic peptides as antigen and immunoblotting procedures. These and other antibodies will be used to characterize the subunit specificity of intermediate filaments of trophoblast cells and to investigate the possible role of different domains of the molecule in the polymerization of cytokeratin filaments. The gene coding for cytokeratin 18 will be isolated and characterized. Regulatory elements necessary for the tissue-specific expression of the protein will be identified by the construction and transfection of fusion genes into expressor and non-expressing human cell lines. These studies whould lead to the complete characterization of a cytokeratin which appears to be a valuable marker of various carcinomas derived from simple epithelial tissues. In addition, the elements necessary for the tissue-specific expression of this gene should be identified and may lead to a better understanding of the relationship of the expression of this protein in different cancers.